(1) Field of the Invention
The present invention relates to an optical pickup device which stores, reproduces and erases information on an optical information storages medium such as an optical disc and an optical card.
(2) Description of the Related Art
As a highly accurate/high capacity storage medium, the applicable range for optical memory technology which uses an optical information storage medium with a pit pattern is expanding to digital audio discs, video discs, document file discs, data files and so on. In optical memory technology, information is stored and reproduced with high accuracy and high reliability in an optical information storage medium using a minutely focused optical beam. These storage and playback processes depend entirely on an optical system. The basic functions of the optical pickup device, which is the main section of the optical system, are divided into condensing which forms diffraction limit microspots, controlling focal points in the optical system, tracking control for the optical system and detecting pit signals. These functions are realized by combining each type of optical system with a photoelectric conversion detection protocol according to objective and application. In recent years, in order to miniaturize and make the optical pickup device thinner, diffraction elements (holograms) are utilized (see for example, Patent Document 1).
FIG. 1 is a diagram which shows the structure of an optical pickup device which utilizes a conventional diffraction element. The optical pickup device 1000 shown in FIG. 1 stores, reproduces and deletes information in the optical information storage medium 1106. For example, the optical information storage medium 1106 is a Compact Disc (CD), a Digital Versatile Disc (DVD), a Blu-ray Disc (BD) or a High Definition DVD (HD-DVD). The optical pickup device 1000 includes a light source 1101A and 1101B, photo detectors 1102A, 1102B, 1102C and 1102D, a diffraction element 1103, an optical path combining unit 1104 and a light condensing unit 1105.
The light source 1101A is a light source which emits light at a first wavelength (for example, blue light) 1120. The light source 1101B is a light source which emits a light (for example, a red-colored light) 1121 at a second wavelength longer than the first wavelength, and a light (for example, an infrared light) 1122 at a third wavelength longer than the second wavelength.
The light 1120 at the first wavelength released from the light source 1101A is reflected by the optical path combining unit 1104, condensed by the light condensing unit 1105, irradiated onto the optical information storage medium 1106 and reflected by the optical information storage medium 1106. The light reflected by the optical information storage medium 1106 is condensed by the light condensing unit 1105, passes through the optical path combining unit 1104, enters into the diffraction element 1103 and is diffracted such that the light enters into the photo detectors 1102A and 1102B.
A light 1121 at the second wavelength released from the light source 1101B is condensed by the light condensing unit 1105, irradiated onto the optical information storage medium 1106 and reflected by the optical information storage medium 1106. The light reflected by the optical information storage medium 1106 is condensed by the light condensing unit 1105, enters the diffraction element 1103, and is diffracted such that the light is introduced into the photo detectors 1102A and 1102C. The light 1122 at the third wavelength is released from the light source 1101B, condensed by the light condensing unit 1105, irradiated onto the optical information storage medium 1106 and reflected by the optical information storage medium 1106. The light reflected by the optical information storage medium 1106 is condensed by the light condensing unit 1105, enters the diffraction element 1103, and is diffracted such that the light is introduced into the photo detectors 1102A and 1102D.
In other words, when the light diffracted to the right side of the figure by the diffraction element 1103 is defined as −(minus), and the light diffracted to the left side as +(plus), out of the diffracted light at the first through third wavelengths, +primary diffracted light enters the photo detection device 1102A. For −primary diffracted light, diffracted light at the first wavelength enters the photo detector 1102B, diffracted light at the second wavelength enters the photo detector 1102C and diffracted light of the third wavelength enters into the photo detector 1102D. A signal is outputted according to the amount of received light from the photo detectors 1102A through 1102D respectively.    Japanese Unexamined Patent Application Publication No. 2001-176